1. Field of the Invention
The present invention relates to a method for making molten metal by melting reduced metal which is produced in a direct reduction furnace, wherein melting is performed in a melting furnace located in the close vicinity of the direct reduction furnace. Herein, reduced metal means metal in the form of lumps, powder, or the like containing metallic iron, metallic nickel, metallic chromium, metallic cobalt, or a mixture of these substances obtained by direct reduction of raw materials in the form of lumps, powder, or the like containing, for example, iron oxides, nickel oxide, chromium oxide, cobalt oxide, or a mixture of these substances. Examples of the molten metal include molten pig iron, molten steel, and molten ferroalloys.
2. Description of the Related Art
With respect to making of molten steel, instead of a conventional blast furnace-converter method, a method has been widely used in which a direct reduction furnace and an electric furnace are placed side by side in a mini-mill and reduced iron produced in the direct reduction furnace is immediately melted and smelted in the electric furnace to produce molten steel.
When the reduced iron produced in the direct reduction furnace is transferred to the electric furnace, one of the biggest problems is how to prevent reoxidation. In order to prevent reoxidation during the transfer, either one of the following two methods has been mainly used: (1) a method in which a cooling zone is provided in a direct reduction furnace so that high-temperature reduced iron is sufficiently cooled close to normal temperature by an inert gas before being discharged to the air; and (2) a method in which high-temperature reduced iron produced in a direct reduction furnace is briquetted by high-temperature press molding using a briquetting machine unit, followed by quenching using water. That is, safe transferring is performed by cooling reduced iron close to normal temperature at which the reduced iron is not reoxidized even in the air.
However, in the methods described above, a large cost for equipment is required, and, additionally, since high-temperature reduced iron having a large amount of sensible heat is cooled and is then reheated in an electric furnace to be converted into steel, a great amount of energy is wasted and the power consumption in the electric furnace is large, resulting in a high cost of molten steel.
Therefore, in order to use the sensible heat of high-temperature reduced iron effectively, a proposal has been made in which high-temperature reduced iron is transported, without being cooled, through a closed pipe by a pneumatic transport system to a melting furnace downstream (Japanese Unexamined Patent Application Publication No. 4-361921).
Another proposal has also been made in which high-temperature briquettes (high-temperature reduced iron) discharged from a direct reduction furnace are transported via a traveling grate conveyor and are cooled by circulating an inert gas so as to be in contact with the high-temperature briquettes while the sensible heat of the high-temperature briquettes is recovered, and the recovered energy is used by a heat exchanger to preheat combustion air, a process gas, etc., and heat recovery is performed again by using the inert gas cyclically (Japanese Unexamined Patent Application Publication No. 56-163209).
However, with respect to the proposals described above, the cost of equipment is increased because of complex facilities, and the operating cost is also increased because of the electric power of boosters for circulating the gas for the pneumatic transport system and the inert gas. As a result, it is not possible to take full advantage of the utilization of the sensible heat of the high-temperature reduced iron, and the costs for the reduced iron are still high.
On the other hand, in order to effectively use waste and the like generated in steel mills, an attempt has been made, in which the waste and the like containing iron oxides, nickel oxide, chromium oxide, cobalt oxide, or a mixture of these substances, to which a carbonaceous material is added as necessary, is agglomerated, and the resultant agglomerates are reduced by heating in a rotary hearth direct reduction furnace to produce reduced metal, and then the reduced metal is melted in a melting furnace, such as an electric furnace or a converter, to recover the metallic portion. Such a process has been employed by many steel mills. The amount of reduced iron produced by this process is often relatively small compared to the amount of reduced iron produced by the conventional direct reduction furnace. Therefore, it is not possible to employ the method of effectively using the sensible heat of reduced metal by the pneumatic transport system or the circulation of the inert gas because the cost of equipment for unit production volume of the reduced metal is excessive in relation to the cost-saving effect by the recovery of sensible heat. Under these circumstances, a plurality of heat-resistant containers are prepared, and a predetermined amount of high-temperature reduced metal is put in each container. The containers containing the high-temperature reduced metal are transported to a melting furnace in sequence and the reduced metal is loaded into the melting furnace. The empty containers are brought back to the direct reduction furnace to be reused. Moreover, in order to save on the cost of equipment, the containers are often manually transported using forklifts and cranes.
As the heat-resistant container, which must retain heavy and high-temperature reduced iron for a long period and must withstand handling during transportation, a steel container with refractory lining, a thick steel sheet provided with a cooling fin and a stiffening rib, or the like is used. Consequently, the cost of the container is high, handling of the container is not easy due to the large weight of the container, and the costs of maintenance against the abrasion of the refractory lining, the steel sheet, etc., are also high.
Furthermore, a complex operation, such as reversing of the container to load the reduced metal into the melting furnace, or a complex structure, such as a container in which the bottom is constructed so as to be opened and closed is required.
Although it depends on the direct reduction process to be employed, at least approximately 2 to 3 percent by mass of reduced metal powder is contained in the reduced metal due to generation of powder when the direct reduction furnace is loaded with the raw material and within the direct reduction furnace. The melting yield may be decreased and the working environment may be degraded due to flying of the powder when the reduced metal is loaded into the melting furnace.
It is an object of the present invention to provide a method for making molten metal with low cost, in which expensive facilities are not required, the melting yield is not decreased, or the working environment is not degraded.
In a method for making molten metal according to the present invention, wherein reduced metal which is produced in a direct reduction furnace is melted in a melting furnace located in the close vicinity of the direct reduction furnace to produce the molten metal, the method includes the steps of putting the reduced metal into a metallic container, and loading the container containing the reduced metal into the melting furnace.
In the method for making molten metal, preferably, the container is a steel drum.
Preferably, the method for making molten metal further includes, before the step of loading the container containing the reduced metal into the melting furnace, a step of cooling the surface of the container so that the surface temperature of the container is 500xc2x0 C. or less.
Preferably, the method for making molten metal further includes, before the step of putting the reduced metal into the container, a step of cooling the reduced metal to 500xc2x0 C. or less.
In accordance with the present invention, since reduced metal which is produced in a direct reduction furnace is put into a metallic container while retaining the high temperature of the reduced metal, and the container containing the reduced metal is loaded into a melting furnace, the container is not required to be heat-resistant and strong enough to be used for a long period of time, and a metallic container having a simple structure with a relatively small thickness can be used. Consequently, since the weight of the container is decreased and a complex operation, such as reversing the container at the melting furnace, is not required, the handling load is significantly decreased. The maintenance against the abrasion of the refractory lining, the steel sheet, etc., is not required. That is, melting can be performed by simple facilities while retaining the sensible heat of the reduced metal in the melting furnace, resulting in a reduction in cost. Additionally, since the container containing the reduced metal is loaded into the melting furnace, powder is prevented from flying, thus improving the melting yield and maintaining the satisfactory working environment. Additionally, since the container itself is used as the raw material for melting, the melting yield is further improved.
If a steel drum is used as the metallic container, the cost of the container can be saved. In particular, if a waste drum is used, the cost of the container is not substantially required, and the waste drum, which is currently disposed of as it is, can be reused. Thus, the production cost of molten metal is further decreased.
That is, if a waste drum is loaded into a melting furnace as it is, it floats in molten metal with the majority of the waste drum being not immersed in the molten metal because of its hollowness. As a result, the melting efficiency of the waste drum is significantly lower than the melting efficiency of usual scraps. Additionally, since the steel drum is usually composed of a thick steel sheet so as to withstand handling during transportation and to resist corrosion due to long-term storage, it is difficult to decrease the volume by crushing with a commonly used press.
Therefore, in order to reuse the steel drum, for example, a system of reusing a steel drum disclosed in Japanese Unexamined Patent Application Publication No. 10-57928 may be used, in which, after a steel drum is crushed into scraps with a four axial shredder, the coating material, etc., is burnt and removed by heating in a rotary kiln, and the scraps are then pelletized by a pelletizer. However, use of this system has not been implemented because of high cost of equipment in conjunction with many steps involved.
In contrast, in the present invention, since the waste drum containing reduced metal is loaded into the melting furnace, melting proceeds with a considerable portion of the whole waste drum being immersed in molten metal, and thereby a high melting efficiency is achieved.
Furthermore, by cooling the surface of the container to 500xc2x0 C. or less, the strength of the metallic container is not substantially decreased, and the container is not deformed when it is transported to the melting furnace. Even when the surface of the container is coated, the coating material is not volatilized or burnt, thus further improving the working efficiency and environment.
Alternatively, instead of cooling the surface of the container, by cooling the reduced metal to 500xc2x0 C. or less, obviously the same effect is displayed.